Environmental pollution, such as air pollution, is a serious problem that is particularly acute in urban areas. Much of this pollution is produced by exhaust emissions from motor vehicles. NOx gases, which are present in automotive exhaust pollution, are known to cause various environmental problems such as smog and acid rain. The term NOx actually refers to several forms of nitrogen oxides such as NO (nitric oxide) and NO2 (nitrogen dioxide). Nitrogen oxide (NOx) contained in exhaust gas can directly effect the human body. NOx and its emission concentrations in various exhaust gases also contribute to the formation of “acid rain” and photochemical smog. Hence, it is necessary to remove NOx from exhaust gas.
Selective Catalytic Reduction (SCR) is a technique that is used to inject urea—often a liquid-reductant agent—into an exhaust stream of a diesel engine, which is then adsorbed onto the surface of a catalytic converter. In an SCR system, urea is used as a reductant that is converted to ammonia which reacts in the presence of a catalyst to convert NOx to nitrogen and water which is then expelled through a vehicle tailpipe. Precise ammonia and NOx measurements are required to develop and characterize optimal catalyst strategies in order to prevent excess ammonia emissions or un-reacted NOx emissions. Note that the term “ammonia slip” refers to excessive ammonia emission which in practice may be caused by exhaust gas temperatures that are too cold for the SCR reaction to occur (such as during a cold start), or if the urea injection device feeds too much reductant into the exhaust gas stream for the amount of NOx produced by the engine combustion.
A technology that can immediately control the NH3 feed rate according to the load change, fluctuation in NOx concentration, and so forth, is therefore needed in order to realize high-efficiency NOx removal without leaving un-reacted NH3. A measuring technology with a high-speed response capable of simultaneous and continuous measurement of NOx and NH3 would be indispensable. Sensors designed for NOx or NH3, however, are often significantly cross-sensitive to each other. Distinguishing these components is therefore critical to successfully controlling an SCR device. It is believed that the control of SCR devices would benefit from the simultaneous measurements of NOx and NH3.
One approach for the development of simultaneous NOx/NH3 sensor in exhaust gas involves the use of two identical sensors for measuring NOx and NH3 by splitting the exhaust path in two and running each path through a different catalyst prior to entry into the respective sensor. This technique is suitable for stationary power plant application but is very expensive to implement and takes up a great deal of space and is thus not suitable for automotive applications.
In an effort to address the foregoing difficulties, it is believed that two sensors with dissimilar sensitivities and cross sensitivities to NOx and NH3 can be combined and a decoupling observer algorithm applied for simultaneously measuring NOx and NH3 in diesel exhaust as described in greater detail herein.